drop down list ???

I am having drop down list  control .. .. 

i done like below 

dropdownlist1.items.add("-select-")

later

//Creating list item based on loantype1 & roi1 //Here i am using db connections to fetch data required & using while(dr.read() ) & making use of duplicate variables loantype & roi i made them ..

listitem L = new listitem (loantype1,roi1)

//add list item into drop down list 

I made way complex to understand my doubt sorry for that .. i just need to know how come an object "L" of listitem passed into dropdownlist1.items.add(L); then after i am getting correct data into dropdownlist .. but how ??

TY